As pointed out in our U.S. Pat. No. 3,806,336 issued Apr. 23, 1974, it is known that the iron/chromium alloy system has, in its composition diagram, a "limit of metastability" or "spinodal" which is thermodynamically defined as the locus of disappearance of the second derivative of the Helmholtz free energy with respect to the composition of the system. When a high-temperature composition, which is of a homogeneous single-phase structure (.alpha.-phase), of the alloy is brought within the spinodal in a lower temperature range, it is transformed into a separated two-phase structure (.alpha..sub.1 +.alpha..sub.2), the phase separation being called "spinodal decomposition".
The decomposed alloy has a periodic microstructure generally of the order of hundreds of angstroms and which consists of two composition-modulated isomorphous phases in which one phase (.alpha..sub.1) is in the form of an iron-rich fine precipitate uniformly distributed in the other phase (.alpha..sub.2) which is chromium-rich and forms the matrix. Since in such a microstructure the first phase (.alpha..sub.1) is magnetic or ferromagnetic and the second phase (.alpha..sub.2) is nonmagnetic or paramagnetic, there results a single-domain structure whereby a highly retentive magnetic body can be obtained.
U.S. Pat. No. 3,806,336 has pointed out that the iron/chromium alloy of spinodal decomposition type, when it contains cobalt, optionally also with one or both of molybdenum and tungsten in the proportions set forth therein, represents an improved magnetic-material system whose magnetic retentivity and magnetic energy product are comparable with or generally even higher than those of "Alnico" (iron/aluminum/nickel/cobalt) alloys which have hitherto been the mainstay of the magnetic industry. It has been taught that addition of silicon up to a certain proportion moderates heat-treatment conditions required to accomplish the spinodal decomposition of the alloys without materially decreasing the desirable magnetic properties attainable therewith. U.S. Pat. No. 3,954,519 issued May 4, 1976 discloses an improved iron-chromium base spinodal decomposition-type magnetic alloy which by weight consists of essentially 3 to 30% cobalt, 10 to 40% chromium, 0.2 to 5% one or both of niobium and tantalum, 0 to 5% aluminum and the balance iron. U.S. Pat. No. 4,171,978 issued Oct. 23, 1979 discloses another improved iron-chromium base spinodal decomposition-type magnetic alloy which by weight consists of essentially of 3 to 30% cobalt, 10 to 40% by weight chromium, 0.1 to 15% vanadium and the balance iron. The art has also recognized that these magnetic alloys may contain additions of one or more of manganese, nickel, copper, zirconium and aluminum in a small proportion. In brief, all of the iron-chromium base spinodal decomposition type magnetic alloys referred to hereinbefore contains cobalt as one essential element of the ternary alloy system and it has commonly been believed that any such alloy to be practical must contain a substantial amount of cobalt as the essential third element to the iron-chromium binary alloy system. The recent instability of raw cobalt supply and distribution and the extreme rise of its price in recent years have, however, made these iron-chromium-cobalt magnetic alloys less than satisfactory and not as economical than originally expected.